This invention relates to improved copolymers useful as biocompatible materials. Specifically, this invention relates to copolymers useful as ophthalmic lenses, for example, contact lenses.
Early hard contact lenses were produced from polymethyl methacrylate (PMMA) or cellulose acetate butyrate (CAB). Rigid, gas permeable (RGP) contact lenses formed of silicone-containing copolymers offered various advantages over PMMA and CAB lenses (particularly increased oxygen permeability). Another example of an RGP material is a copolymer comprising a silicone-containing monomer and methyl methacrylate.
A newer class of copolymers for silicone-containing RGP lenses is itaconate ester copolymers. U.S. Pat. Nos. 4,152,508 (Ellis et al.), 4,330,383 (Ellis et al.) and 4,826,889 (Ellis et al.) disclose copolymers for contact lenses prepared from: a monofunctional siloxanyl ester monomer; an itaconate ester; an ester of a monohydric or polyhydric alkanol or phenol and a (meth)acrylic acid; a crosslinking agent; and preferably a hydrophilic monomer.
Known copolymers for RGP lenses also include copolymers of fluorinated itaconate esters, such as the copolymers disclosed in U.S. Pat. Nos. 4,686,267 (Ellis et al.) and 4,996,275 (Ellis et al.), which are prepared from a fluorinated itaconate ester and an ethylenically unsaturated organosiloxane.
Other examples of itaconate ester copolymers for RGP lenses are disclosed in the following U.S. Pat. Nos. 4,602,074 (Mizutani et al.); 4,508,884 (Wittmann et al.); 4,743,667 (Mizutani et al.); 4,826,936 (Ellis); and 4,861,850 (Novicky).
As disclosed in U.S. Pat. No. 4,152,508, the itaconate ester provides rigidity, hardness and some degree of wettability to the resultant copolymer. But the inclusion of an itaconate ester tends to make the resultant copolymers more brittle. This illustrates a major challenge in the development of new ophthalmic lens materials: that adding a given monomer to a polymer mix with the goal of attaining a particular characteristic may inherently include other less desirable characteristics. For example, silicone-containing monomers may desirably contribute rigidity, while undesirably decreasing toughness.
Certain multifunctional organosiloxanes have been described as useful for adding higher impact strength and reducing brittleness of itaconate RGP copolymers. U.S. Pat. No. 4,826,936 describes a class of multifunctional organosiloxanes having such properties.
Silsesquioxane macromers are silicon-containing cage molecules based upon the structure of oxygen and silicon tetrahedrons. Silsesquioxane macromers can be polymerized to produce heat-resistant materials such as high-temperature plastics. Examples of silsesquioxane-containing materials are disclosed in U.S. Pat. Nos. 5,412,053; 5,484,867; and 5,589,562 (all to Lichtenhan et al.). While bulky materials containing silicon and oxygen have been of interest to improve the oxygen permeability of biocompatible materials such as contact lens materials, those materials are markedly different than the heat-resistant materials that are the subject of ongoing research relating to silsesquixane macromers.
Thus it would be desirable to improve the oxygen permeability of biocompatible materials without sacrificing other essential properties. In particular, it would also be desirable to provide a biocompatible material that is optically clear, and also has excellent machinability and toughness.
The present invention relates to silsesquioxane-containing copolymers that are useful as biocompatible materials. The copolymers of the invention are the polymerization product of a mixture comprising:
(a) a monomer selected from the group consisting of itaconates, (meth)acrylates, fumarates and styrenes;
(b) an ethylenically unsaturated organosiloxane monomer; and
(c) a POSS compound containing at least one substituent group comprising a polymerizable activated unsaturated group.
The monomers useful for polymerizing with POSS compounds in accordance with the invention include any ethylenically unsaturated monomer, for example, alkyl methacrylates such as methyl methacrylate and neopentyl methacrylate, as well as aromatic methacrylates such as benzyl methacrylate.
Halogenated methacrylates and itaconates are also useful monomers in the present invention. Examples include fluoroalkyl methacrylates such as hexafluoroisopropyl methacrylate and trifluoroethyl methacrylate, and halogenated itaconates such as bis (1,1,1,3,3,3-hexafluoro-2-propyl) itaconate.
Aromatic ring compounds may also be useful as monomers in the present invention. Examples of suitable aromatic monomers include alkyl substituted styrenes such as t-butyl styrene. Other useful monomers include vinyl lactams and n-vinyl pyrrolidone.
Hydrophilic monomers such as (meth)acrylic acid and (meth) acrylamides such a N,N-dimethylacrylamide are also useful in the invention.
Monomers useful in the present invention may also contain two or more functional groups of the type listed above.